I was expecting a totally different method. There is a fire thing technique that uses long flexible strips of bamboo or vines to generate heat in a notched hearth board by rapidly pulling them back and forth. You can get a steel cable as from a bicycle brake cable to do the same thing. It will get smoking hot.The key is to arrange your materials in such a way that the hot dust collects in one place to form an ember like a bow drill does.
The video in the link shows what you are describing with the bamboo/vines. I got the impression from the video that the rattan, bamboo "cord" they were using was what gave up the fibers to create the embers. You're saying its just the opposite or that it could be just the opposite. Something to keep in mind.
The video also showed the guy using a piece of something hollow, reed, bamboo, something, to blow the bird's nest into flames. Having lost my eyebrows once in a sudden flame up, I like that solution.
Ah, yeah. The methods at the link are quite a bit different from the one depicted in the post. Looks like a 2 man drill rather than a fire thong.
I do think more material from the hearth board is converted to make a coal than from the thong strips themselves. The reason being that the the thong strip moves across the hearth board and the friction is distributed along it's length, but the part of the hearth board in contact with the thong is small and it is always under friction.
Another way of saying that is that the hearth board has an inch wide area in contact with the thong, but the thong might have a 10" long area in contact with the hearth board as it moves back and forth. So the thong has a chance to cool more so than the hearth does.
